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Re: [TowerTalk] update: grounding questions: Ufer, strap, exothermic we

Subject: Re: [TowerTalk] update: grounding questions: Ufer, strap, exothermic welds
From: Jim Lux <>
Date: Mon, 18 Apr 2011 20:55:12 -0700
List-post: <">>
On 4/18/11 8:00 PM, Gary Schafer wrote:
> Hi Mat,
> Copper strap is better than wire as it has lower inductance and lower
> resistance.
True when you're talking about  the same cross sectional area of 
copper..  Inductance doesn't change all that much, but resistance 
changes a lot.

2/0 is about 0.36 inches in diameter  (0.10 square inch, roughly)   to 
get the same copper area in Georgia Copper's "standard" thickness 
(0.022"), you'd need a 5" wide piece.

If you start looking at skin depth, though.. at 1 MHz (a nice round 
number for lightning discharges).. the skin depth in copper is about 
0.0026 inches.
That 0.36 inch diameter is about 1" in circumference, so the "effective 
cross section" for 1 MHz is about 0.0026 square inches.   A 1/2" wide 
piece of copper strap would have the same effective cross section.

Copper stuff is basically priced by the pound, so for the same AC 
resistance, the strap is 1/10th the price of a round wire.  And if you 
go up in frequency, the trade is even more attractive: you could use an 
even thinner strap, as long as the width of the strap is more than half 
the circumference.  So, for a HF antenna ground, or a conductor carrying 
high RF currents...

However, that 1/2"x0.022" strap is mechanically a lot less rugged than a 
3/8" diameter copper wire, and when it comes to handling high peak 
currents with high di/dt, you want mechanically rugged.

> Do not run any wires or strap out of the concrete base below grade.
> Someone mentioned that they ran wires thru the concrete base and out the
> side below grade to conveniently connect the radials to the tower.
Indeed, that would be a bad plan.. below grade penetration = microscopic 
cracks for water to infiltrate in

> Radials should be bonded directly to each tower leg with a longer radius
> from tower to earth over the concrete base. Don't make a neat sharp bend
> over the edge of the concrete. No "ground rings" around the tower.
Again, this is more for mechanical reasons (with electromagnetic forces 
from a lightning pulse) than in an attempt to reduce inductance. The 
inductance of a half or quarter turn of wire is not much different than 
the inductance of the same length of wire in a straight line.  However, 
a sharp bend has larger magnetic forces on the wires (the forces try to 
make the radius of a bend larger).


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